9,279 research outputs found
Exotic paired phases in ladders with spin-dependent hopping
Fermions in two-dimensions (2D) when subject to anisotropic spin-dependent
hopping can potentially give rise to unusual paired states in {\it unpolarized}
mixtures that can behave as non-Fermi liquids. One possibility is a fully
paired state with a gap for fermion excitations in which the Cooper pairs
remain uncondensed. Such a "Cooper-pair Bose-metal" phase would be expected to
have a singular Bose-surface in momentum space. As demonstrated in the context
of 2D bosons hopping with a frustrating ring-exchange interaction, an analogous
Bose-metal phase has a set of quasi-1D descendent states when put on a ladder
geometry. Here we present a density matrix renormalization group (DMRG) study
of the attractive Hubbard model with spin-dependent hopping on a two-leg ladder
geometry. In our setup, one spin species moves preferentially along the leg
direction, while the other does so along the rung direction. We find compelling
evidence for the existence of a novel Cooper-pair Bose-metal phase in a region
of the phase diagram at intermediate coupling. We further explore the phase
diagram of this model as a function of hopping anisotropy, density, and
interaction strength, finding a conventional superfluid phase, as well as a
phase of paired Cooper pairs with d-wave symmetry, similar to the one found in
models of hard-core bosons with ring-exchange. We argue that simulating this
model with cold Fermi gases on spin dependent optical lattices is a promising
direction for realizing exotic quantum states.Comment: 10 pages, 12 figure
Electrodynamics of Fulde-Ferrell-Larkin-Ovchinnikov superconducting state
We develop the Ginzburg-Landau theory of the vortex lattice in clean
isotropic three-dimensional superconductors at large Maki parameter, when
inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov state is favored. We show that
diamagnetic superfluid currents mainly come from paramagnetic interaction of
electron spins with local magnetic field, and not from kinetic energy response
to the external field as usual. We find that the stable vortex lattice keeps
its triangular structure as in usual Abrikosov mixed state, while the internal
magnetic field acquires components perpendicular to applied magnetic field.
Experimental possibilities related to this prediction are discussed.Comment: 5 pages, 1 figur
Finite temperature phase diagram of a polarized Fermi gas in an optical lattice
We present phase diagrams for a polarized Fermi gas in an optical lattice as
a function of temperature, polarization, and lattice filling factor. We
consider the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO), Sarma or breached pair
(BP), and BCS phases, and the normal state and phase separation. We show that
the FFLO phase appears in a considerable portion of the phase diagram. The
diagrams have two critical points of different nature. We show how various
phases leave clear signatures to momentum distributions of the atoms which can
be observed after time of flight expansion.Comment: Journal versio
How many phases meet at the chiral critical point?
We explore the phase diagram of NJL-type models near the chiral critical
point allowing for phases with spatially inhomogeneous chiral condensates. In
the chiral limit it turns out that the region in the mean-field phase diagram
where those phases are energetically preferred very generically reaches out to
the chiral critical point. The preferred inhomogeneous ground state in this
vicinity possibly resembles a lattice of domain wall solitons. This raises the
question of their relevance for the phase diagram of QCD.Comment: 7 pages, 1 figure; v2: minor corrections, as published in PR
What do we really know about the constraints and enablers of physical activity levels in young children?
Ample evidence exists to support the benefits of habitual physical activity levels on long term health and well-being for adults, youth and, more recently, children. However information about factors that impact on young children’s choices to be active or inactive is limited because the accurate measurement of physical activity in young children is difficult. Consequently, many statements found in the literature about young children’s physical activity levels are not founded on empirically validated information. Some conclusions are extrapolated to younger children from findings in studies of older children.
This paper is based on an extensive review of literature and the outcomes of a National Summit held in Fremantle, WA in November 2001. It seeks to identify those key early life experiences and context factors that research has confirmed create the child who is attracted to physically active play rather than sedentary play. The implications of these findings for future research and intervention programs are presented
Neutrality of a magnetized two-flavor quark superconductor
We investigate the effect of electric and color charge neutrality on the
two-flavor color superconducting (2SC) phase of cold and dense quark matter in
presence of constant external magnetic fields and at moderate baryon densities.
Within the framework of the Nambu-Jona-Lasinio (NJL) model, we study the
inter-dependent evolution of the quark's BCS gap and constituent mass with
increasing density and magnetic field. While confirming previous results
derived for the highly magnetized 2SC phase with color neutrality alone, we
obtain new results as a consequence of imposing charge neutrality. In the
charge neutral gapless 2SC phase (g2SC), a large magnetic field drives the
color superconducting phase transition to a crossover, while the chiral phase
transition is first order. At larger diquark-to-scalar coupling ratio
, where the 2SC phase is preferred, we see hints of the
Clogston-Chandrasekhar limit at a very large value of the magnetic field
(G), but this limit is strongly affected by Shubnikov de
Haas-van Alphen oscillations of the gap, indicating the transition to a
domain-like state.Comment: 19 pages, 7 figures, Matches with the published versio
Nonuniform states in noncentrosymmetric superconductors
In noncentrosymmetric crystals, nonuniform superconducting states are
possible even in the absence of any external magnetic field. The origin of
these states can be traced to the Lifshitz invariants in the free energy, which
are linear in spatial gradients. We show how various types of the Lifshitz
invariants in noncentrosymmetric superconductors can be derived from
microscopic theory.Comment: 7 page
Thermal conductivity in a mixed state of a superconductor at low magnetic fields
We evaluate accurate low-field/low-temperature asymptotics of the thermal
conductivity perpendicular to magnetic field for one-band and two-band s-wave
superconductors using Keldysh-Usadel formalism. We show that heat transport in
this regime is limited by tunneling of quasiparticles between adjacent vortices
across a number of local points and therefore widely-used approximation of
averaging over circular unit cell is not valid. In the single-band case, we
obtain parameter-free analytical solution which provides theoretical lower
limit for heat transport in the mixed state. In the two-band case, we show that
heat transport is controlled by the ratio of gaps and diffusion constants in
different bands. Presence of a weaker second band strongly enhances the thermal
conductivity at low fieldsComment: 7 pages, 1 figure, discussion of the clean case and discussion of
experiment adde
Vortex Viscosity in Magnetic Superconductors Due to Radiation of Spin Waves
In type-II superconductors that contain a lattice of magnetic moments,
vortices polarize the magnetic system inducing additional contributions to the
vortex mass, vortex viscosity, and vortex-vortex interaction. Extra magnetic
viscosity is caused by radiation of spin waves by a moving vortex. Like in the
case of Cherenkov radiation, this effect has a characteristic threshold
behavior and the resulting vortex viscosity may be comparable to the well-known
Bardeen-Stephen contribution. The threshold behavior leads to an anomaly in the
current-voltage characteristics, and a drop in dissipation for a current
interval that is determined by the magnetic excitation spectrum.Comment: 4 pages, 1 figur
Transport in a Dissipative Luttinger Liquid
We study theoretically the transport through a single impurity in a
one-channel Luttinger liquid coupled to a dissipative (ohmic) bath . For
non-zero dissipation the weak link is always a relevant perturbation
which suppresses transport strongly. At zero temperature the current voltage
relation of the link is where and
denotes the compressibility. At non-zero temperature the linear
conductance is proportional to . The decay of
Friedel oscillation saturates for distance larger than
from the impurity.Comment: 4 page
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